These are marked off from each other by intervening constrictions, and are named the fore-brain or prosencephalon, the mid-brain or mesencephalon, and the hind-brain or rhombencephalonthe last being continuous with the medulla spinalis.

The fore-brain or prosencephalon consists of: (1) the diencephalon, corresponding in a large measure to the third ventricle and the structures which bound it; and (2) the telencephalon, comprising the largest part of the brain, viz., the cerebral hemispheres; these hemispheres are intimately connected with each other across the middle line, and each contains a large cavity, named the lateral ventricle.

In its early embryonic condition it consists of three hollow vesicles, termed the hind-brain or rhombencephalon, the mid-brain or mesencephalon, and the fore-brain or prosencephalon; and the parts derived from each of these can be recognized in the adult (Fig. 677).

The cephalic end of the neural groove exhibits several dilatations, which, when the tube is closed, assume the form of three vesicles; these constitute the three primary cerebral vesicles, and correspond respectively to the future fore-brain (prosencephalon), mid-brain (mesencephalon), and hind-brain (rhombencephalon) (Fig. 18).

_ -- About the middle of the first month of intra-uterine life the prosencephalon bends acutely forward over the end of the notochord and sends out from its base a series of processes, which ultimately blend to form the face (Fig. 231).